There has come into general use a class of synthetic polyurethane which is formed by the reaction of two highly reactive chemicals, typically an organic resin and a polyisocyanate. These chemicals are relatively stable when alone, but when mixed in the proper portion, react within a very few seconds to form a polyurethane foam.
Depending upon the specific composition of the chemicals used to make the polyurethane, the mixture can be made to foam as it sets, so as to make the polyurethane foam suitable for use in a variety of applications, including insulation and foam-in-place cushioned packaging material. The fast reaction and setting of the chemicals to form the polyurethane foam is highly desirable from the standpoint of high production rates of the foam, but reaction of the chemicals inside the dispensing equipment can clog and jam the internal parts of the dispensing equipment, rendering the equipment inoperative.
To alleviate some of the problems of mixing and dispensing of polyurethane foam precursor chemicals, several different approaches to the design of dispensing equipment for polyurethane foams have been suggested.
A type of polyurethane foam dispensing equipment in wide use is exemplified in U.S. Pat. No. 3,263,928. The device disclosed in this patent includes a mixing chamber assembly having a Teflon core circumferentially and axially contained within a metallic housing. The Teflon core has an axial bore therethrough with individual inlets therein for the injection of the reactive chemicals from respective sources of supply into the bore and a valving rod reciprocally movable within the bore. The Teflon core is maintained under a compressive force by a threaded nut disposed at the rear of the mixing chamber core to keep an interference fit between the valving rod and the Teflon core to maintain proper sealing of the inlets with the valving rod.
The valving rod is connected to an air piston which is actuated by means of a trigger. When the trigger is depressed, the valving rod is reciprocated back to uncover the inlets, thereby allowing entry of the reactive chemicals into the bore for mixing and dispensing out the front of the bore. After the desired quantity of the mixture has been dispensed, the rod is reciprocated forward to close off the ports to prevent further entry of the reactive chemicals into the bore. The valving rod is then moved still forward to the end of the bore so as to purge mixed chemicals which remain in the mixing chamber bore.
This design has met with a limited degree of success, but has many drawbacks. Despite the interference fit between the valving rod and the bore, as the valving rod is actuated a number of times, a gradual build-up of reacted polyurethane material on the valving rod occurs. Even though Teflon is an inherently low friction material, as polyurethane adheres to the valving rod, the friction and/or adhesion which develops between the valving rod and the core may increase to the point where the valving rod becomes jammed in the bore and cannot be reciprocated by the air cylinder. When this happens, the dispensing equipment is rendered inoperative and must be disassembled and cleaned to free the valving rod. In time, the Teflon core itself may become unserviceable due to wear. In this case, after removal of the valving rod from the Teflon core, the Teflon core itself must be removed from the housing and replaced, which is a laborious process. During such repair, the apparatus is unusable.
Other proposals for minimizing the potential for jamming of the valving rod in the bore are disclosed in U.S. Pat. Nos. 3,687,370 and 4,023,733. These dispensing devices have a Teflon mixing chamber core with a bore therethrough fitted into a metallic housing. The disclosed devices also include a valving rod fit tightly into the bore and reciprocal to selectively seal or open the ports, similar to the operation shown in U.S. Pat. No. 3,263,928, except without means for axially compressing the Teflon core. The devices disclosed in U.S. Pat. Nos. 3,687,370 and 4,023,733, however, are provided with a reservoir containing a solvent material behind the mixing chamber. When the rod is reciprocated to the rearward position, the back of the rod is bathed in the solvent, which acts to prevent the reaction of the precursor chemicals and dissolve built-up polyurethane, which is said to reduce the tendency of the valving rod to jam in the Teflon mixing chamber. However, jamming can still occur, and such jams necessitate disassembly and cleaning of the equipment. In some cases, the jamming may be so serious as to require removal and replacement of the Teflon core from the housing, which is a time consuming and laborious process requiring special equipment. During such repair or replacement, the apparatus is unusable. Furthermore, the solvent itself is usually toxic.
U.S. Pat. Nos. 4,469,251 and 4,568,003 disclose a dispensing device similar to the device disclosed in U.S. Pat. No. 3,263,928, but which include proposals for simplifying replacement of the mixing chamber assembly and valving rod in the event jamming occurs.
These patents teach use of dispensing apparatus having a detachable mixing chamber assembly. The mixing chamber assembly has a Teflon core having a bore and a valving rod in an interference fit in the bore. The Teflon core is axially and circumferentially retained in a non-deformable housing, which is also part of the detachable mixing chamber assembly. This housing axially and radially contains and compresses the Teflon core to maintain the interference fit. These patents further teach use of non-deformable inserts in the inlets to maintain the shape of the inlets during use. Belleville spring washers are also provided at one end of the housing to maintain the Teflon under a compressive force. This, it is said, maintains the interference fit between the valving rod and the bore so as to maintain an effective seal between the valve and the bore to prevent leakage. However, jamming may still occur, and in this event, the mixing chamber, and hence the device, becomes unusable.
In an effort to reduce the time during which the device of U.S. Pat. Nos. 4,469,251 and 4,568,003 is unusable, the disclosed mixing and dispensing apparatus has two separable portions. One portion includes a mixing chamber assembly, including the non-deformable housing, Teflon core, non-deformable inserts, valving rod and Belleville washers; and another portion includes means for reciprocating the valving rod. When the mixing chamber assembly becomes jammed or otherwise fails to function, the entire mixing chamber assembly, including the non-deformable housing, Teflon core, non-deformable inserts, valving rod and Belleville washers, are removed from the other portion of the apparatus and replaced with a fresh assembly.
While ostensibly an improvement over the type of dispenser disclosed in U.S. Pat. No. 3,263,928; 3,687,379 and 4,023,733, the apparatus disclosed in these later patents has some disadvantages. Use of a reciprocal rod in the bore of the Teflon core as the valving means for the inlets is undesirable. Because the chemicals are maintained under pressure directly adjacent the valving rod, unless the core is kept under a high compressive force to maintain a tight interference fit between the valving rod and the bore, the chemicals have a tendency to leak around the valving rod when the valving rod is in the forward position, causing premature reaction of the chemicals and thus contributing to jamming and clogging of the inlets. Furthermore, the relatively high compression necessary to maintain sealing of the valving rod in the Teflon core increases the friction between the valving rod and the Teflon core, and thus the force required to reciprocate the valving rod in the bore. Still further, the compression on the core has a tendency to cause the non-deformable inserts in the inlets to back away from the rod to the extent permitted by the non-deformable housing. This can potentially allow the Teflon material of the core itself to extrude underneath the discharge end of the insert to occlude the inlets.
Teflon is also a material which exhibits a tendency to "cold flow". Thus, Teflon will flow when subjected to a compressive force. The tight compressive force necessary between the mixing chamber core and the associated valving rod has a tendency to compress the Teflon radially inwardly and facilitate jamming. In time, after a number of valve rod reciprocations, the inner surface of the core is shaved somewhat destroying the integrity of the tight interference fit. This shaving problem is especially acute when the valve rod is tilted or non-aligned within the core.
Furthermore, the mixing chamber assembly with its non-deformable housing, Belleville washer assembly for maintaining the Teflon core under compression, Teflon core, non-deformable inserts and valving rod is relatively expensive. Discarding the entire mixing chamber assembly is undesirable because of the expense, particularly since the failure is usually confined to only the Teflon core itself and valving rod (which are themselves relatively inexpensive). However, because the Teflon core is tightly contained within the circumferential housing, removal and replacement of the Teflon core and valving rod alone from the housing is difficult and requires special equipment.
U.S. Pat. No. 4,377,256 to Commette et al. discloses an apparatus for dispensing a mixture of mutually reactive liquids, such as a hardenable plastic foam. The apparatus includes a purging rod which reciprocates within a mixing chamber. At the rear end of the mixing chamber is formed a counter bore which partially receives a scraper. The scraper includes a cylindrical outer sleeve with a converging front wall that converges into a sharp edge. A plastic bushing is maintained within the cylindrical outer sleeve by way of a washer force fitted into the rear portion of the cylindrical sleeve. The rear edge of the cylindrical sleeve is pressed tightly against a recess formed in the gun block. Within the recess formed in the gun block is also positioned a felt wiper saturated with a solvent material.
Commette et al. rely on the scraper as well as the solvent to avoid build up of material on the purging rod. The use of solvent wipers necessitates additional maintenance requirements that are often neglected resulting in the sticking of the purging rod. Also, Commette et al., in partially relying on the use of the solvent, suggests the use of tolerances that are not sufficiently low enough to avoid build up on the purging rod. This results in an abrading of the plastic insert due to solidified build up on the rod. The degrading of the insert also allows the fluid in the chamber to leak past the sharp edge of the scraper, past the insert and out through the back of the mixing chamber.
Further, the force fitting of the rear washer and the high degree of compression forcing the scraper into the shoulder of the counter bore formed in the gun block prevents a slight shifting of the scraper components which slight shifting can be useful in avoiding sticking of the purging rod and providing dynamic stability. Also, the use of the sharp edge scraper and the low axial length to width ratio of the scraper and plastic insert in Commette is not entirely sufficient in avoiding misalignment of the purging rod and the resultant scraping of the interior of the nozzle.